1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a leaky-mode buried-heterostructure laser.
2. Brief Description of the Prior Art
Heretofore, as a semiconductor laser wherein light generated and propagated in a p-n junction is subjected to the laser oscillation by a resonator structure, the double heterostructure which exhibits a good optical confinement in the junction region has been often employed. The double heterostructure is such that a semiconductor layer (for example, GaAlAs layer) having a low refractive index and a high band-gap energy is joined on both sides of a semiconductor region (for example, GaAs layer) serving as an active layer in which the laser oscillation is carried out.
The double heterostructure has differences in the refractive index in a direction perpendicular to the hetero-junction, but it exhibits no difference in the refractive index in a direction parallel to the hetero-junction. It is the mesa-stripe type double heterostructure that has been devised in order to bestow a refractive index variation in the lateral direction. This structure is obtained in such a way that crystal surfaces parallel to the hetero-junctions of the double heterostructure are subjected to the mesa-etching, to form a striped mesa and to make the active layer striped. Therefore, an external space or a different substance adjoins directly onto both lateral sides of the active layer, and differences in the refractive index are exhibited also in the lateral direction. This structure is further classified into the high-mesa type and the low-mesa type in dependence on the depth of the mesa-etching, and when compared with the prior-art double-heterostructure laser, it has the merits that the optical confinement is effective and that the spreading of current in the lateral direction does not occur. These facts have been published in, e.g., "Appl. Phys. Letters," vol. 20, p 344-345, May 1972, or "IEEE J. Quantum Electronics," vol. QE-9, No. 2, Feb 1973.
Further, when a semiconductor layer of low refractive index is grown once more around the striped region subjected to the mesa-etching, there is obtained a structure in which the striped active region of high refractive index is surrounded by the semiconductor material of low refractive index. With such a structure, the active region is enclosed with the semiconductor material having a similar refractive index, and hence, its optical characteristics are improved. The details of the structure are disclosed in, e.g., Japanese Laid-Open of Patent Application Nos. 50-10985 and 50-10986. A semiconductor laser wherein stripes for preventing the spreading of current are disposed over and under the active layer is disclosed in U.S. Pat. No. 3,849,790.
In the semiconductor lasers of the wave guiding structures thus far described, however, the diffraction phenomenon of light arises in consequence of the confinement of light, and laser beams are emitted at considerably wide angles of divergence. The beam divergence is approximately 8.degree.-30.degree. though, of course, it varies depending on the structure of the particular semiconductor laser.
The intensity distribution of a laser beam which is emitted from a semiconductor laser has a Fourier transform relationship with the light intensity distribution within the laser. Accordingly, the reduction of the beam divergence is possible in principle by rendering the area of an optical confinement region large. However, when the optical confinement region is expanded, laser oscillations in lateral higher-order modes become prone to develop. The development of the laser oscillations in the higher-order modes results in spreading the divergence angle of the laser beam as a whole. For this reason, it is very difficult to make the beam divergence less than 8.degree. in the semiconductor lasers of the prior-art structures.